1. Field of the Invention
This invention relates to a separate type smoke sensor which emits lights to a reflecting plate disposed at a certain distance from the sensor, receives reflected light from the reflecting plate and outputs a sense signal if the level of received light is reduced to a predetermined threshold value by smoke entering an observed region. More particularly, this invention relates to a separate type smoke sensor which can cancel the influence of a shielding object to obtain the true quantity of reflected light from a reflecting plate no matter what the reflectivity of the shielding object, and which, therefore, can correctly determine whether or not there is a fire.
2. Description of the Related Art
A sensor arranged as Japanese Patent Provisional Publication No. 296641/92 (Japanese Patent Application No. 146460/91) is known as a conventional photoelectric smoke sensor of this kind.
A reflecting plate is placed across an optical axis of light emitted from a light emitting portion. Light reflected by the reflecting plate is received by a light receiving portion. If the light is intercepted by intrusion of smoke, a received light level at the light receiving portion is changed. This change is detected and the received light level and a predetermined threshold value are compared to determine whether or not there is a fire.
FIG. 26(a) schematically shows the construction of a conventional separate type photoelectric smoke sensor. As can be understood from FIG. 26(a), in the conventional separate type photoelectric smoke sensor, light from a light emitting device 102 provided in a sensor main unit 100 is collimated into a projected beam 106 by a lens 104, the beam 106 passes across an observation space, and the direction of traveling of the beam 106 is turned by 180.degree. by a retroreflection mirror (reflecting plate) 101. A turned beam 107 is condensed by a light receiving lens 105 and received by a light receiving device 103. If smoke 110 generated by a fire exists in the observation space, the quantity of light of the received beam is reduced. A corresponding received light level is compared with a threshold value to recognize the fire. For example, if the level of a received light signal, which is normally 100 mW, is reduced to 50 mW, a fire signal is generated.
If, as shown in FIG. 26(b), a shielding object 121 other than smoke enters the observed region of the thus-constructed fire sensor under ordinary observation conditions, the sensor may erroneously determine that there is a fire by detecting a reduction in the level of a received light output from the light receiving portion. In such a situation, a person in charge goes to the place there the fire sensor is set, confirms the existence of the shielding and removes the shielding object to restore the ordinary observation conditions.
There is also a possibility of occurrence of a non-observing condition if the observation light is intercepted by a shielding object. A sensor capable of outputting a warning signal when the level of the received light signal becomes extremely low has also been proposed to avoid such a situation.
In the above-described separate type photoelectric smoke sensor, the received light level at the light receiving portion is reduced in the case of shielding of shielding object 121 having a low reflectivity. In such a case, a trouble detection operation may be performed to enable the above-described method to be used as an immediate means. However, if the shielding object has a high reflectivity, light from the light emitting portion is reflected by the shielding object 120 and received by the light receiving portion. In such a case, the same received light level as that under the normal condition can be obtained and there is a risk that the sensor may determine that the state of the observed area is normal even if there is a fire. A region between the shielding object 120 and the reflecting plate 101 cannot be observed and there is a risk of warning failure.
In some or many cases, this kind of sensor is placed close to a ceiling of a building. However, pipings and ducts are usually laid in the vicinity of building ceilings. If a place in which a separate type photoelectric smoke sensor is set is such that a pipe or a duct is within a limit radial range of the sensor, this type of sensor must be replaced with a different type of sensor in order to avoid warning failure due to reflection light from such a shielding object, even if it is effective to use the separate type photoelectric smoke sensor in other respects.